Optical components, in particular components employing optical fibers doped with rare earth metal ions, are widely used in telecommunication systems. A major application is in signal amplifiers which employ fluorescent ion emission for amplification of a signal. The ion emission occurs within the same operating wavelength region as the signals. Pump energy excites the rare earth metal ion causing it to fluoresce and thereby provide optical gain.
Glasses, doped with a rare earth metal ion and pumped with appropriate energy, exhibit a characteristic, fluorescence intensity peak. The evolution of telecommunication systems, with their increasing demands for bandwidth, has created a need for a rare-earth-doped, amplifier material having the broadest possible emission spectrum in the wavelength region of interest. It is a purpose of the present invention to meet this need.
The bandwidth of a fluorescent intensity curve is, rather arbitrarily, taken as the full width half maximum (FWHM) of the curve in nanometers wavelength. This value is the lateral width of the curve at one half the maximum intensity, that is, at one half the vertical height of the peak of the curve. Unfortunately, many glasses, that exhibit a fluorescence in an appropriate region, exhibit a rather narrow bandwidth. It is a further purpose of the invention to provide a family of glasses that exhibit a relatively broad bandwidth.
It is well known that glasses doped with erbium can be caused to emit fluorescence in the 1520-1560 nm. region. This enables a signal operating in this wavelength range to be amplified. The significance of the 1550 wavelength in optical communication has led to extensive studies regarding the behavior of erbium as a rare earth metal dopant in glasses. It has also led to the study of a variety of glasses as the host for the erbium ion.
The low phonon energy of tellurite glasses can lead to long emission lifetimes at certain pump wavelengths. As an example, erbium in tellurite glasses exhibits long tau-32 (980 emission) lifetimes relative to silicates. The long emission lifetimes at the pump wavelength can reduce the efficiency of an amplifier or laser because of excited state absorption. A practical 980 pumping scheme can be obtained by co-doping glasses with components having phonon overtones that are resonant with the energy difference between 980 and 1530 nm. Such components include H.sub.2 O, B.sub.2 O.sub.3, P.sub.2 O.sub.5.
For certain applications, long, erbium, tau-32 emission lifetime is desirable. These include long-band amplifiers, where pumping directly into the ground-state absorption does not impact noise figure; also tilt-free amplifiers, in which 980 and 1480 pump lasers are combined to dynamically adjust the gain without affecting the shape of the gain spectrum.
It is also known that glasses with moderately low maximum phonon energies, when doped with thulium, can display fluorescence in the vicinity of 1450 nm. Although this wavelength lies outside of the currently used telecommunications band, it still lies within the transparency window of most commercial optical fiber. With the ever-increasing demand for useful bandwidth, there will be a need for additional amplifier devices that operate over the remaining portions of this window that are not covered by erbium.
It is known, for example, that, as the concentration of a rare earth metal ion, such as erbium, is increased, the optical gain increases up to a certain point. Beyond this point, the fluorescent signal is quenched, and the optical gain decreases. This phenomenon is considered to result from the dopant, rare earth metal ions interacting with each other in a manner commonly referred to as clustering. It is another purpose of the invention to provide a family of glasses which is readily capable of dissolving erbium ions, and which exhibits a broad bandwidth indicating that clustering is inhibited.
It has been reported that certain tellurite glasses, doped with erbium ions, provide a very broad, erbium emission band in the 1540 nm. region of the spectrum. Glass compositions were not reported, but other studies indicate that the glasses are alkali-alkaline earth-zinc-tellurite glasses.
Most tellurite glass families require the presence of at least about 50% TeO.sub.2 in the glass compositions to permit glass formation. It has been thought that a broader range of the glass forming oxide, TeO.sub.2, should provide a variety of structural motifs of the network-forming TeO.sub.x species. Such species include polymerized, bipyramidal TeO.sub.4 groups at high TeO.sub.2 content and isolated pyramidal TeO.sub.3 groups at low TeO.sub.2 content. The diversity of species should yield a greater diversity of structural sites for the incorporation of dopant ions, such as erbium ions. This avoids the ions clustering and becoming ineffective for fluorescent emission and consequent amplification. Also, the diversity of dopant sites should give rise to broadened emission spectra.
It is, then, a basic purpose of the present invention to provide a family of tellurite glasses that can be melted over an extensive compositional region, and in which compounds of rare earth metals, such as erbium, are readily soluble.